High-voltage batteries may be used to energize electric machines in a variety of different systems. For instance, output torque from an electric machine may be used to power an input member of a transmission in a plug-in vehicle, i.e., a vehicle having a battery pack that may be recharged via a charging outlet or other offboard power supply. The individual cells of a battery pack gradually age and degrade over time. As a result, battery performance parameters such as open circuit voltage, cell resistance, and state of charge may change relative to calibrated/new values. Battery degradation is therefore typically monitored by a designated controller in order to estimate the amount of electrical energy remaining in the battery pack. Electric vehicle range estimates can be generated from the estimated electrical energy and thereafter used for effective route planning, and/or to execute automatic powertrain control actions.
Several factors can contribute to battery degradation and shorten battery life. For instance, battery packs that are maintained at a high state of charge level tend to degrade much faster than battery packs maintained within a lower, more optimal state of charge range. Higher battery charging currents and temperatures can also shorten battery life. Battery packs of the types typically used in plug-in vehicles are trending toward larger sizes suitable for longer all-electric driving distances, in some cases well over 200 miles on full charge. However, range anxiety and other factors such as time constraints, personal driving habits, and a limited appreciation for battery physics may lead to preferred battery charging habits that can shorten battery life. For instance, if a given operator's normal daily electric driving range is 30-50 miles in a vehicle having a fully-charged electric operating range of 200 miles, the act of fully charging the battery pack at every charging event will result in maintenance of a high state of charge throughout the duration of ownership of the vehicle. This in turn may reduce battery life, and can adversely affect the accuracy of electric range estimates over time.